Atom Defender

How to play

Defend your ship from waves of stray particles. Between waves you revise and answer three questions; each right answer charges a proton blast that clears the screen.

On a computer

Left and right arrow keys to move (your ship fires on its own), Space for a proton blast

On a phone

Drag the ship or hold ◀ ▶; tap Blast to clear the screen

What you’ll revise

Class 9 Science: Atoms and Molecules, Structure of the Atom

1. Wave 1 · Atoms and molecules

   • Mass is neither created nor destroyed in a chemical reaction: the law of conservation of mass.
   • A compound always has the same elements in the same ratio by mass: water is always 1 : 8 hydrogen to oxygen.
   • Atomic mass is measured in u. 1 u is one-twelfth the mass of a carbon-12 atom.

2. Wave 2 · Valency and formulae

   • Valency is an atom’s combining capacity: hydrogen 1, oxygen 2, nitrogen 3, carbon 4.
   • Ions carry a charge: Na⁺ and Cl⁻ combine to make sodium chloride, NaCl.
   • Criss-cross the valencies to write a formula: aluminium (3) and oxygen (2) give Al₂O₃.

3. Wave 3 · Inside the atom

   • Electrons are negative, protons are positive and neutrons have no charge.
   • Rutherford’s gold-foil experiment showed a tiny, dense, positive nucleus at the centre of the atom.
   • Atomic number = number of protons. Isotopes have the same atomic number but different mass numbers.